Art. LX.—On the Formation of Timaru Downs.
[Read before the Hawke's Bay Philosophical Institute, 12th July, 1886.]
Dr. von Haast, in his work on the “Geology of Canterbury and Westland,” p. 367, ascribes the formation of the Timaru plateau to a sub-aerial origin, and compares its structure to the loëss (or loam) deposits of China, the Rhine, and Danube, as described by Baron von Richthofen, the eminent German traveller and geologist, who, he says, “has shown in his last publications
that the loëss in China could only be of sub-aerial origin, deposited by apnais, which at the present time are still at work in forming that rock. Atmospheric currents, together with the growth of grass and other vegetation, during an untold number of years, are the principal agencies by which the loëss has been deposited. In the first instance, rain water, running down the more or less steep slopes of the country, carries with it fine particles, which are partly retained by the grass or amongst its roots; whilst the wind, blowing across the land, takes up a great amount of fine sediment, afterwards also partly caught and retained by the grass. However, a third and most important agent is to be found in the roots of the plants themselves decaying, and thus raising the ground. There is a peculiar vertical capillary texture observable in the true loëss, deriving, doubtless, its origin from the decaying of numberless rootlets during many past generations of grasses.” Dr. Haast goes on to state, “during the Great Glacier Period of New Zealand, beginning towards the end of the pliocene and ending in the post-pliocene period, during quaternary and recent times, the loëss-beds have gone on accumulating steadily so as to reach such a considerable thickness, as we find them, amongst other localities, as the lower slopes of Banks Peninsula, and on the Timaru plateau.”
This view has been opposed by Professor Hutton, who, in an article on the silt deposit at Lyttelton, laid before the Philosophical Institute of Canterbury,* clearly shows that those deposits do not belong to the loëss formation. After weighing all the evidence he could obtain, he arrives at the conclusion “that the evidence in favour of the marine origin of this deposit preponderates enormously over the evidence in favour of its sub-aerial origin,” including in this judgment the Timaru formation. Not having seen much of Banks Peninsula, I am unable to make any personal remarks on the silt formation there; but with Timaru it is different, as having been resident there for some time, I have had the opportunity to obtain such information as makes me differ from Dr. von Haast as well as from Professor Hutton.
The Timaru Downs are situated to the south of the Canterbury Plains; they are about six miles broad, and extend inland from the sea about ten miles. They consist of gentle undulating country, well adapted for agriculture. The structure of these rolling downs is very peculiar. It is very evident, from abundance of data, that the Canterbury Plains at one time extended all along where these downs now exist, and that actually the plains are there at present (beneath), and that the downs have been built on the plains. The plains beneath the downs have been covered over with beds of dolorite or basalt, and over the
[Footnote] * “Trans. N.Z. Inst.,” vol. xv., p. 411.
dolorite occur beds of fine brownish-yellow material, interlaid with regular streaks of volcanic ash. These beds curve with the hill, and do not occur in flat beds, as in marine deposits. The regular streaks of volcanic ash are very evident in any new cutting, and the pieces of ash can be readily picked out. That dolorite exists below the downs, and resting on the shingle plain, there is ample evidence, as both can be traced up all the deep gullies; and has also been proved from wells that have been sunk, from bores put down, and from quarries. The shingle beneath the dolorite beds shows evidence of having been subjected to great heat, and the dolorite in many places is scoriaceous; in fact, many pieces can be found that could not be distinguished from Auckland (Mount Eden) scoria. Nobody can for a moment doubt but that this dolorite was emitted as lava from some volcano situated above these downs—probably near Mount Horrible. This lava spread over the country in two or three layers, pouring down in ridges. The volcano being spent as to lava, it then, doubtless, belched out ooze and mud, with occasional showers of cinders and ash. The ejected material would overlay and envelope the dolorite beds. There is an excellent section north of Timaru, formed by a railway cutting, showing the dolorite bed, and above it the beds of ooze, with unmistakable layers of cinders (Pl. XXVIII.). In this bed of ooze, deep down, I have found moa bones, but no trace of land or marine shells; and I have not observed the peculiar vertical capillary texture observable in the true loëss, as described by Dr. von Haast. The occurrence of moa bones would tend to prove that these beds were comparatively recently formed, as might be from a sudden volcanic outbreak, and not from a slow formation as that of the loëss, which would take ages, and so reach the time prior to advent of the moa.
I am led the more strongly to uphold the volcanic origin of these downs from having seen very similar formations elsewhere, when there could not be the slightest doubt of their formation. That was in Auckland, during the execution of the Auckland improvement works at Albert Barracks. Heavy cuttings had to be made for the streets, and one of these cuttings went actually into the cinder cone of an old crater. Further away from the cone were similar beds to those at Timaru, with layers of cinders through them.*
There is a peculiar feature in these downs which is a puzzle to all, and that is the occurrence of small lagoons or shallow ponds on the brows of the hills. Almost invariably, as you mount a hill you will find a lagoon on top. Had these lagoons only occurred anywhere else, they would not have caused any
[Footnote] * A drawing of this in section can be seen in the “Trans. N.Z. Inst.,” vol. vii., p. 144.
attention; but, occurring as they do, it does seem strange how they could have been formed. The only suggestion I can offer to explain this circumstance, is that—having adopted the volcanic theory in the formation of these downs—immediately after the lava or dolorite beds were spread over the country, the mud and ooze were deposited on them. The great heat of the beds—greatest when thickest—would for a long time keep the mud boiling, and so a quantity of solfataras or mud volcanoes would be formed, and when the whole cooled a shallow basin would be left where they existed.